Liquid toner based copying machines are described in U.S. Pat. Nos. 4,286,039, 4,411,976, 4,727,394, and 3,900,003, the disclosures of which are hereby incorporated by reference. In these copying machines, an electrostatic latent image is formed on a photoconductive carrier movable into proximity with a developer electrode held at a voltage intermediate the voltages on the carrier representative of background and information portions of the image. Liquid toner, comprising a dielectric carrier liquid containing charged toner particles, is applied between the photoconductive carrier and the developer electrode. The toner particles, being charged, are drawn to and plate out on information portions of the carrier. In regions on the carrier associated with background, toner particles are drawn toward and tend to plate out on the developer electrode as the regions on the carrier associated with background portions of the image move past the electrode. After development of the image, the carrier moves to a transfer station where the developed image is transferred to a receiving sheet. Thereafter, the carrier is cleared of any residual toner particles, charged to a high voltage at a charging station, and moved to an image station where another image is optically projected onto the carrier. The process described above then repeats.
Substantial amounts of toner particles are likely to remain on the developer electrode from a preceding cycle of operations. Such particles will adhere to the developer electrode when the product of the force attracting the particles to the electrode and the coefficient of friction of the particles on the surface of the electrode is greater than the shear force caused by the flow of toner liquid over the electrode, if such flow is present at all. From a practical standpoint, it is well known that toner particles adhere to the developer electrode and must be removed if good copies of images on the carrier are to be obtained.
U.S. Pat. No. 4,168,329 discloses removal of toner particles from a developer electrode by pulsing the electrode with a reverse bias subsequent to development of the image, i.e., during an inter-image interval of operation of the machine. This procedure cleans toner particles from the electrode but results in the deposition of the particles onto the carrier necessitating their removal by a further processing operation. This technique is also disclosed in U.S. Pat. Nos. 4,041,217, 4,168,329, and 4,423,134.
Other techniques are also known, such as utilizing a conductive roller for the developer electrode (U.S. Pat. No. 4,454,833). Toner particles deposited on the roller can be removed by mechanical means, for example, which contact that surface of the roller outside the development region. Other techniques involve placing a high charge on a transverse band on the carrier during the inter-image interval, and passing the charged band over the electrode to deplate toner particles which adhere to the band. This approach, however, also requires further processing of the carrier before the next cycle of operations can begin.
Finally, as disclosed in GB 1,414,335, the developer electrode may be coated with a polymer which inhibits adhesion of toner particles thereto. Polymers disclosed in this patent include a silicone resin, a polytetrafluoroethylene, a polyurethane, a polypropylene, a polyvinyl chloride, a polycarbonate, and a cellulose acetate. According to the patent, the coating has a thickness in the range of from 1-100 microns.
Experiments were performed by the inventors of the present invention to determine if release coatings on developer electrodes have an effect on the image quality. These experiments have established that an electrode coated with a dielectric, such as vinyl polydimethylsiloxane polymer containing a noble metal complex as a catalyst, and a hydrogen functional polymethylsiloxane polymer with thickness of 20 or 40 microns or a polytetrafluoroethylene of 50 micron thicknesses, adversely affect image development. The effect on image development is best demonstrated by producing copies which are uniformly dark. In such case, the optical density of large image areas decrease significantly in the process direction when a development electrode coated in the manner described above is used. This effect is more pronounced for thicker dielectric coatings. This effect is not observed when an uncoated electrode is employed, and is more pronounced for thicker coatings. From this standpoint, coated electrodes known in the prior art and described in GB 1,414,335 are not practical for images which contain large printed areas.
While the techniques of the prior art have been reasonably successful in some respects, all but the approach taken in GB 1,414,335 include a residual cleanup process to remove toner particles that are deplated from the developer electrode before the next copying cycle is carried out. It has also been found that when toners of the type described in example 1 of U.S. Pat. No. 4,794,651 are used with coated developers of the type known in the art, there is a tendency to form a deposit on the developer despite the coating.
It is therefore an object of the present invention to provide a new and improved liquid toner based electro-static imaging machine, and a developer electrode therefor, which eliminates, or substantially reduces, the above-mentioned deficiencies of the prior art.